The variation adds up to $1 billion a year that insurance companies, patients and government health plans are needlessly paying to treat breast cancer, Giordano and colleagues at the MD Anderson Cancer Center report on Monday in the journal Cancer. Patients often get stuck with thousands of dollars in bills that can take years to pay off.

"One of the reasons I started on this project is I had a patient with triple-negative breast cancer," Giordano told NBC News.

"She was just finishing her five-year follow-up after treatment and was transitioning into survivorship. We talked about ordering an additional test. She was reluctant to get it."

"She shared with me that she was still on a payment plan, still trying to pay off the debt from her breast cancer treatment five years earlier," Giordano said.

"There were big variations in the cost of treatments we could prescribe."

"That really shocked me. I didn't know. I think it cured her but it left her with this huge debt that I was completely unaware of."

In fact, the costs can get so high that some patients may skip treatments.

"If they have a higher copay and it costs more, then they are less likely to take it," Giordano said. "There are pretty clear data that show the more it costs, the less people take it."

As a working oncologist, Giordano knew there are almost always a variety of choices for treating breast cancer. She wanted to see if there was an easy way for patients and their doctors to pick the least expensive treatment that would still be effective.

She was stunned at what she found. The team went through insurance claims filed by more than 14,000 breast cancer patients between 2008 and 2012 and found expenses across a single class of drugs varied by as much as $46,000.

"There were big variations in the cost of treatments we could prescribe," Giordano said.

Even more surprising: Some of the more effective and least toxic treatments were, in fact, cheaper.

"There is one chemotherapy regimen called TAC," Giordano said. That's short for Taxotere plus adriamycin plus cyclophosphamide — three different chemotherapy drugs that hit the cancer in different ways.

"It was both one of the highest cost and one of the highest toxicity regimens," she said.

Swapping a related drug called paclitaxel for Taxotere cut both the side effects and the costs, Giordano said. "It's almost $15,000 cheaper," she said.

"I think most people would think those pretty equivalent in terms of efficacy," she added.

But it wasn't easy to find this out.

"For me, working at a hospital, I don't really see that at all. I don't have an easy way to access what the costs would be," she said.

Even treatments that should be cheap varied widely in cost.

"We give a pill called an aromatase inhibitor that blocks estrogen after chemotherapy. There are three different ones that are all approved and they are probably equivalent," Giordano said. These pills reduce the chance that tumors will come back in the other breast.

Many patients would just delay treatment and wait until their cancer progressed, which ends up costing even more to treat. And patients may end up going on Medicaid and costing the state and federal government plan extra money for treating advanced cancer.

Both Giordano and Johnson say it's very difficult to find out ahead of time what each particular insurance plan will pay, and what each hospital, clinic or physician will charge, for various treatments.

"This is everybody's problem," Johnson said. "I know women who have lost their homes because they couldn't pay their medical bills."